We investigate the possible impacts of the
Pacific Decadal Oscillation (PDO) on the occurrence of
weak stratospheric polar vortex (WSV) events in the Northern
Hemisphere winter. WSV events, which are defined
when polar-cap geopotential height anomalies at 50 hPa
fall below the 10th percentile in winter, are observed more
frequently during positive PDO phases than during negative
PDO phases. Additionally, tropospheric wave forcings
that drive WSV events are remarkably different between
the two phases of the PDO. During the positive PDO phase,
the vertical propagation of wavenumber-one waves plays
a predominant role with a rather minor contribution of
wavenumber-two waves. This contrasts with the negative
PDO phase when the WSV events are primarily caused by
wavenumber-two waves. This difference is partly related to
the PDO-induced tropospheric circulation anomalies over
the North Pacific whose zonal wavenumber-one component
constructively (destructively) interferes with climatological
planetary-scale waves during positive (negative) PDO
winters. The predominant wavenumber-two wave forcings
during the negative PDO phase are likely related to
the enhanced tropospheric eddy activity over Alaska that
results from the poleward shift of the Pacific jet in response
to the negative PDO.